1. Field of the Invention
The present invention relates to an LC resonance circuit that is formed as a semiconductor integrated circuit (hereafter referred to as an xe2x80x9cICxe2x80x9d) and the like and is provided with a voltage-controlled variable-capacitance element whose capacitance value changes in correspondence to the level of a control voltage and a voltage-controlled oscillation circuit (hereafter referred to as a xe2x80x9cVCOxe2x80x9d) having this LC resonance circuit.
2. Description of the Related Art
The technologies related to VCOs having an LC resonance circuit in the prior art include that disclosed in the following publication.
Publication: 1999 VLSI Circuits, (U.S.) xe2x80x9cAxc2x130% Tuning Range Varactor Compatible with future Scaled Technologiesxe2x80x9d P. 34xcx9c35
The great progress made in the field of mobile wireless devices and the like in recent years has prompted extensive integration of high-frequency circuits to achieve miniaturization of the devices and a reduction in the power consumption.
A high-frequency circuit for a radio device often employs a VCO as the oscillations source. A VCO is an oscillation circuit whose oscillation frequency changes in correspondence to the level of the control voltage. A VCO manufactured as a MOS-IC for instance, is normally provided with an LC resonance circuit that employs a P/N junction type voltage-controlled variable-capacitance element (also referred to as a xe2x80x9cP/N junction type varactorxe2x80x9d) or a MOS-type voltage-controlled variable-capacitance element (also referred to as a xe2x80x9cMOS-type varactorxe2x80x9d) such as those disclosed in the publication mentioned above, to facilitate the IC manufacturing process.
As explained in the publication, a voltage-controlled variable-capacitance element employed in a VCO in the prior art operates by, in principle, using the change in capacitance between the anode electrode and the cathode electrode caused by changing the voltage applied to a P/N diode, or using the change in capacitance between the gate electrode of a MOS capacitor and the substrate caused by changing the gate voltage applied to the gate electrode of the MOS capacitor. Both of the voltage-controlled variable-capacitance elements pose problems in that since the capacitance changes drastically in correspondence to the change in the control voltage, a large amplitude cannot be set for the control voltage in conjunction with a VCO employing either type of the voltage-controlled variable-capacitance elements and in that the poor linearity of the output frequency relative to the control voltage results in control difficulties.
An object of the present invention is to provide a high-performance LC resonance circuit enabling good control, in which the rate at which the capacitance value of a voltage-controlled variable-capacitance element changes can be set freely in conformance to particulars of design and a VCO that employs this LC resonance circuit, by addressing the problems of the prior art discussed above.
In order to achieve the object described above, a first aspect of the present invention provides an LC resonance circuit comprising a coil connected between a first node and a second node, n (n is a positive integer equal to or larger than 2) variable-capacitance elements, each having a control electrode and connected in parallel between the first node and the second node with the capacitance value thereof caused to change in correspondence to a variable voltage applied to the control electrode, and m (m is a positive integer equal to or larger than 1) means for voltage reduction, each of which is connected to an input terminal at which a control voltage is input and provides a voltage achieved by lowering the control voltage to the control electrode of a respective one of the n variable-capacitance elements or one of (nxe2x88x921) variable-capacitance elements.
In second and third aspects of the present invention, the variable-capacitance elements in the LC resonance circuit in the first aspect of the present invention are each constituted of a MOS-type varactor or a P/N junction type varactor.
In fourthxcx9csixth aspects of the present invention, the means for voltage reduction in the LC resonance circuit in the first aspect of the present invention are each constituted of a MOS transistor which operates by using a forward voltage reduction occurring between a drain electrode and a source electrode in a diode connection in which a gate electrode and the drain electrode are shorted, a diode that operates by using a forward voltage reduction or a diode that operates by using a reverse breakdown voltage.
In the structure described above, the control voltage input to the input terminal is individually lowered by the m means for voltage reduction and the resulting reduced voltages are supplied to the control electrodes of the n variable-capacitance elements. The capacitance value of each variable-capacitance element is determined in conformance to the voltage supplied to its control electrode. Since the n variable-capacitance elements are connected in parallel, the value achieved by adding the individual capacitance values constitutes a combined capacitance value C. Since the coil is connected to the n variable-capacitance elements, the resonance frequency is expressed as f=1/(2xcfx80(LC t)) with L representing the reactance value of the coil.
In seventhxcx9ctwelfth aspects of the present invention, a VCO is provided with an LC resonance circuit in any of the firstxcx9csixth aspects of the invention. As a result, the capacitance values of the n variable-capacitance elements within the LC resonance circuit change as the control voltage input through the input terminal changes and, in correspondence to the changes in the capacitance values, the resonance frequency f (i.e., the oscillation frequency) changes.